Active-matrix EL display devices having EL elements that are arranged in matrix are commercialized and are used for display panels of smartphones, for example. The EL element has an EL layer formed between an anode electrode and a cathode electrode, and emits light by a current or voltage supplied to the anode electrode and the cathode electrode (terminal).
Anode current or cathode current of EL elements, arranged in matrix on a display screen, increases as EL display panel becomes larger. This enlarges a voltage drop in a wiring where the anode current or cathode current flows. The greater voltage drop causes a greater loss in electric power, and deteriorates the EL display panel due to heat. Further, display quality can be degraded because of brightness-shading or cross-talk on the EL display screen.
Japanese Patent Application Publication JP2009-109519A1 describes an EL display device having a common power supply line formed outside an array unit and an auxiliary wiring connected to this common power supply line.
Japanese Patent Application Publication JP2006-039541 describes a structure of supplying a voltage of a printed circuit board to an EL display device via wiring formed in TCP (tricyanopyrroline).
A cathode electrode formed on the display screen of the EL display panel is made of very thin magnesium (Mg)-silver (Ag) so that the cathode electrode can have light transmittance. Therefore, its sheet resistance value becomes larger. If the sheet resistance is large, a current flowing in the cathode electrode causes a large voltage drop, and brightness-shadings or cross-talk can be generated as a result. Further, a current-flow can concentrate in a connecting point where a wiring for supplying cathode voltage to the cathode electrode is connected. As a result, the connection point melts due to overheat or heat generation.
To address this problem, one proposed approach is to form a wiring portion made of low resistance metal material and connect this wiring portion to the cathode electrode. In such case, the wiring portion needs to be formed thick to keep its resistance low for solving the above problem, but if a thick wiring is formed, a size of display area occupying some part of the EL display panel become smaller and a frame of the display panel become larger. On top of that, this structure limits the potential locations for connecting a flexible substrate that supplies the cathode voltage, so that enough numbers of connecting points cannot be expected. As a result, the cathode current can be concentrated in the connecting point and connecting point, which then melt down, and reliability of the wiring can be degraded.